The present invention generally relates to an optical communication system for acquiring an optical link between terminals. More particularly, the present invention relates to an acquisition sensor for such a system including a multi-channel photodetector for use in acquiring an optical link between terminals.
At least two communication terminals are involved in laser communications, a transmitting terminal and a receiving terminal. The transmitting terminal transmits the optical signal (such as laser energy) which is received by the receiving terminal. The receiving terminal receives the optical signal with a detector such as a photodetector.
One of the difficulties in long-distance laser communication between a transmitting terminal and a receiving terminal is the initial alignment of the optical transmitting source and the receiving detector of the two communication terminals. For example, if the optical transmitting source is a laser, the laser from the transmitting terminal must be pointed so that the laser is incident on the detector of the receiving terminal. When the separation between satellites is great (for instance, thousands of kilometers), this initial alignment and acquisition may be quite challenging. In addition to the wide separation between terminals, the laser beam itself may be quite narrow, further adding to the challenge. The narrowness of the laser beam arises because of the power constraints inherent in satellite communications. Wider beams require more power which in turn adds to satellite weight, cost and size.
Many prior systems used one or more laser beacons to align communication satellites with respect to each other. Multiple laser beacons increase cost in terms of both the size and weight of the satellite and the power consumption of the beacon. Other systems relied on hyper-accurate initial positioning, which may not be achievable when the separation between terminals is large and may be easily disrupted by spacecraft jitters.
Thus, a need has long existed for an acquisition system and sensor that minimizes power consumption and additional weight and size while providing reliable and fault tolerant acquisition within a short time.
One object of the present invention is to provide an optical acquisition sensor for use in an acquisition system that eliminates the laser beacon signal that must be provided in several prior systems.
Another objective of the present invention is to minimize the cost, complexity, size, weight, and power consumption of the hardware used to provide an acquisition system and sensor.
One or more of the foregoing objects are met in whole or in part by the inter-satellite optical link acquisition sensor of the present invention. The present invention provides an acquisition sensor and system for acquiring an optical beam transmitted by a source located in a region of uncertainty. A source (transmitted) optical signal is scanned and the acquisition sensor is employed on a receiving terminal to look for the scanning beam. Based on the information from the acquisition sensor, the Field of View (FOV) and region of uncertainty of the receiver can be adjusted accordingly to establish the communication link.
The acquisition sensor includes a multi-channel photodetector, channel circuitry, a threshold circuit, a comparator and filtering. The received optical signal is converted to an electrical signal and supplied to channel circuitry. The channel circuitry includes a threshold circuit supplying a threshold signal. The received electrical signal is compared to the threshold signal and, if the received signal exceeds the threshold, a hit is detected.
These and other features of the present invention are discussed or apparent in the following detailed description of the preferred embodiments of the invention.